There has hitherto been made a trial of improving mechanical characteristics of a resin by compositing an inorganic oxide such as silica as a filler with a resin. As the method of compositing the filler with the resin, a method of mixing a dispersion containing an inorganic oxide dispersed in water and/or an organic solvent with a resin is usually used. By mixing the dispersion with the resin using various methods, an inorganic oxide particle composite plastic comprising inorganic oxide particles composited as a second phase can be prepared.
On the other hand, as a substrate for flat panel displays (FPD) such as a liquid crystal display (LCD), a plasma display (PDP), or an electroluminescence display (EL), a glass substrate has often been used. This glass substrate has such a problem that it is not suited for weight saving since it is easily broken and cannot be bent, and also has a large specific gravity. Thus, there has often been made a trial of using a plastic substrate having flexibility in place of the glass substrate.
Examples of characteristics required to the plastic substrate for a flat panel display (FPD) include transparency, a refractive index, and mechanical characteristics.
As an inorganic oxide filler for improving the refractive index of the plastic, fine oxide particles made of zirconia and titania are used as a high refractive index filler.
A dispersion containing the inorganic oxide filler in an aqueous solvent or an organic solvent is developed so as to composite the inorganic oxide filler with the resin, and an improvement in the refractive index of the resin is now studied.
As an example of compositing, there is proposed a high refractive index/high transparency film having a thickness of several microns using a zirconia particle composite plastic obtained by compositing zirconia particles having a particle diameter of 10 to 100 nm with a resin (see, for example, Patent Document 1).
As a light emitting element in optical semiconductor devices such as an optical pickup used in a CD, a CD-ROM, a CD-Video, an MO, a CD-R, and a DVD, and various display devices and display equipment, there has widely been used a light emitting diode (LED) which emits light in a visible region, an ultraviolet region or an infrared region by using a forwardly biased pn-junction region as a light-emitting region and recombining electrons and holes in the light-emitting region.
In this light emitting diode, a LED chip obtained by laminating gallium nitride-based compound semiconductors is loaded on a lead frame and the LED chip is electrically connected to the lead frame, and the LED chip is sealed with a resin which exerts both a protective function and a lens function.
In the light emitting diode, when a gallium nitride (GaN)-based compound semiconductor is used as the light emitting layer, the optical refractive index of the light emitting layer is about 2.
As the resin used for sealing, transparency, mechanical strength, and tenacity are required. Therefore, as a resin suited for these requirements, a silicone resin having a refractive index of about 1.4 is widely used (see, for example, Patent Documents 2 and 3).
The resin is composited with an inorganic oxide such as silica as a filler so as to improve mechanical characteristics of the resin.
The method of compositing the resin with the filler includes, for example, a method of dispersing the filler in the resin by mixing a dispersion containing an inorganic oxide in water and/or an organic solvent with the resin using various methods.
In an LED, a semiconductor device is sealed with a transparent sealing resin such as an epoxy resin or a silicone resin so as to protect the semiconductor device. However, there was such a problem that the sealing resin causes yellowing and luminance of the LED decreases since energy emitted from the LED increases with the increase of requirements of realization of a short wavelength and a high luminance of the LED.
When a silicone resin is used as a sealing resin of an LED, the silicone resin is excellent in heat resistance and light resistance and is insufficient in adhesion with the semiconductor device, and also has a low refractive index, thus causing a problem that efficiency of light extraction from the LED decreases.
As the epoxy resin, a bisphenol A type epoxy resin (an epi-bis type resin) and a cresol novolak type epoxy resin are used. The epoxy resin has a property capable of easily absorbing ultraviolet light since it has a benzene ring, namely, an unsaturated bond.
Therefore, the epoxy resin had such a problem that a radical generated by energy of absorbed ultraviolet light may cause oxidation of the epoxy resin, resulting in yellowing.
As a means for solving such a problem, there is proposed a hydrogenated epoxy resin in which an aromatic ring of a bisphenol A type epoxy resin or a cresol novolak type epoxy resin is directly hydrogenated, namely, hydrogen is bonded with the aromatic ring. However, when the aromatic ring is hydrogenated, an unsaturated bond in the epoxy resin decreases and, therefore, light resistance of the hydrogenated epoxy resin is improved, whereas, heat resistance of the hydrogenated epoxy resin deteriorates. Thus, there is proposed a hydrogenated epoxy resin whose heat resistance is improved by mixing the hydrogenated epoxy resin with an acid anhydride, a curing accelerator, and an antioxidant, and optimizing the mixing ratio (see, for example, Patent Document 4).
There has hitherto been made a trial of improving mechanical characteristics of a resin by compositing an inorganic oxide such as silica as a filler with the resin. As the method of compositing the filler with the resin, a method of mixing a dispersion containing an inorganic oxide dispersed in water and/or an organic solvent with a resin is usually used. By mixing the dispersion with the resin using various methods, an inorganic oxide particle composite plastic comprising inorganic oxide particles can be prepared.
In the field of flat panel displays (FPD) such as a liquid crystal display (LCD), a plasma display (PDP), an electroluminescence display (EL), and a surface electric field display (SED), a trial of using various plastic materials in place of a glass substrate, which has conventionally been used, has been made. It becomes possible to solve problems of the glass substrate, such as easiness of breakage, processability, and weight, by using the plastic material.
On the surface of the flat panel display, functional films of various plastic films such as an anti-reflective (AR) film, an anti-glare (AG) film, and a hard coat (HC) film are laminated so as to improve visibility and to prevent a surface flaw. Examples of important characteristics required to such a plastic film include transparency, a refractive index, and mechanical characteristics. When the refractive index is improved, a composite plastic film obtained by compositing a plastic film with an inorganic oxide filler having a high refractive index, for example, zirconia (ZrO2) or titania (TiO2 is used).
The method of compositing the inorganic oxide filler with the plastic is roughly classified into the following methods.
(1) Method of Kneading an Inorganic Oxide Filler into a Plastic
Examples of this method include (a) a method of dispersing an inorganic oxide filler in a resin monomer and polymerizing or polycondensing the resin monomer to obtain an inorganic oxide filler-containing plastic film, and (b) a method of dispersing a liquid resin material an inorganic oxide filler, forming into a film and curing a resin material to obtain an inorganic oxide filler-containing plastic film.
As an example of the inorganic oxide filler-containing plastic film, there is proposed a zirconia particle-containing polyester film in which abrasion resistance of the surface is improved by dispersing zirconia particles having a particle diameter of 0.005 to 0.3 μm in a polyester (see, for example, Patent Document 5).
(2) Method of Forming a Film Containing an Inorganic Oxide Filler on a Plastic Film
This method is a method of applying a coating material containing an inorganic oxide filler and a binder component on a plastic film and curing the binder component to form a film. As the binder component, an inorganic material such as silica produced by a sol-gel method, and a resin material such as polyester or polyether are used.
As an example of this film, there is proposed a high refractive index/high transparency zirconia particle composite plastic film having a thickness of several microns obtained by compositing zirconia particles having a particle diameter of 10 to 100 nm with a plastic (see, for example, Patent Document 1).
As an optical transparent plastic material, polycarbonate (PC), polymethyl methacrylate (PMMA), and a cyclic olefinic resin have widely been used, heretofore. However, when the transparent plastic material is applied to optical lenses for projectors, optical lenses and substrates for pickups of optical disks, and substrates for flat panel displays (FPD), PC has large fringence, and also PMMA has large water absorptivity and insufficient heat resistance.
A cyclic olefinic resin, which can solve these problems, had such a problem that a flaw may be formed on the surface since it is inferior in scratch resistance.
A method of forming a hard coat layer on the surface is proposed so as to improve scratch resistance of the surface of the optical transparent plastic material, and this method is widely used. A high refractive index hard coat layer is used in prevention of interference fringe of a plastic lens and a hard coat layer and applications such as a hard coat for an anti-reflective film (see, for example, Patent Documents 6 and 7).
(Patent Document 1)
    Japanese Unexamined Patent Application, First Publication No. 2005-161111(Patent Document 2)    Japanese Unexamined Patent Application, First Publication No. 2005-105217(Patent Document 3)    Japanese Unexamined Patent Application, First Publication No. 2004-292779(Patent Document 4)    Japanese Unexamined Patent Application, First Publication No. 2005-68234(Patent Document 5)    Japanese Unexamined Patent Application, First Publication No. Hei 05-171012(Patent Document 6)    Japanese Unexamined Patent Application, First Publication No. 2004-1393(Patent Document 7)    Japanese Unexamined Patent Application, First Publication No. Hei 11-302597